Oxazoline/carbene ligands

The relationship between selectivity vs. ligand topography was determined for some ligands based on the x-ray crystal structure of the corresponding iridium complexes." " " The stmctures of the respective oxazoline-carbene ligands 599a-d are shown in Figure 8.24." ... 

The combination of the Herrmann s carbene ligand 58 and the Gade s NHC family 66 resulted in a new chiral N-heterocyclic carbene 69 (Scheme 47) [ 127]. The coupling strategy allows the free combination of oxazoline substituents in... 

Finally, two chiral monodentate N-heterocyclic carbene ligands that contain an oxazoline unit have been reported. Glorius et al. reported the synthesis of the imidazolium salts 76 by cyclizing the corresponding bisoxazolines 75 (Scheme 50) [133]. 

Glorius F, Altenhoff G, Goddard R, Lehmann C (2002) Oxazolines as chiral building blocks for imidazolium salts and N-heterocyclic carbene ligands. Chem Comm 2002 2704-2705... 

Bis-oxazolines (Box), azabis-oxazolines (azaBox) and pyridine bis-oxazoline (PyBox) with a C2 symmetry form the great majority of various oxazoline-derived ligands which are used in immobilized catalytic cyclopropanation reactions (Scheme 36). For these reactions, immobilized ruthenium and copper complexes which are able to form carbene intermediates have been tested. Most of the time, the reaction involved styrene and ethyl diazoacetate (EDA). However other alkenes (diphenyl ethylene. ..) and other diazo compounds (i.e. tertiobutyl and ethyl 2-phenyl diazoacetate) have also been investigated (Scheme 36). 

Bidentate N-Heterocyclic Carbene Ligands Incorporating Oxazoline Units I 327... 

Asymmetric versions of the cyclopropanation reaction of electron-deficient olefins using chirally modified Fischer carbene complexes, prepared by exchange of CO ligands with chiral bisphosphites [21a] or phosphines [21b], have been tested. However, the asymmetric inductions are rather modest [21a] or not quantified (only the observation that the cyclopropane is optically active is reported) [21b]. Much better facial selectivities are reached in the cyclopropanation of enantiopure alkenyl oxazolines with aryl- or alkyl-substituted alkoxy-carbene complexes of chromium [22] (Scheme 5). 

Several other enantiomeric ally pure bidentate ligands have been screened using [Ir (cod)Cl]2, including bis-oxazolines [48,49], amino oxazolines [50],Ai-heterocyclic carbenes [51], and diamines [52, 53]. Examples where the enantioselectivity was in excess of 90% ee include the use of ligands 45,46, and 47 [54-56]. Selected examples of the use of these ligands with [Ir(cod)Cl]2 are given in Scheme 12 for the reduction of ketones 48, 50, and 52. 

Catalytic asymmetric cyclopropanations via carbene transfer to alkenes were reviewed by Singh and co-workers in 1997," Doyle and Protopopova in 1998," and mostly recently by Doyle in 2000." The reaction can be catalyzed by copper," rhodium," and other metals." Bis(oxazolines) are known to be among the most effective ligands for this cyclopropanation reaction (see Chapter 9). 

Mononuclear oxazolines have been less studied. Moderate enantioselectivities (up to 60%) were obtained using PyrOx as ligands for the copper(l)-catalyzed carbene-transfer reaction of ethyl diazoacetate to styrene." " " However, the diastereo-selectivities (cis/trans) in these reactions were generally poor. 

---